EFFECTS OF LIGHT AND DARKNESS ON THE EYE 



527 



Text-fig. 3 Reconstruction of dorsal ganglia and eyes of dark adapted eyes. 

 Note the elongation of eyes and their laterally directed axes. 



CONCLUSIONS 



1 . Stimulation by light results in a contraction of the accessory 

 cell or pigment cell. 



In sustained darkness the cytoplasmic lamellae of the pigment 

 cell open up or move apart, resulting in the expansion of the cell. 



2. The three cytoplasmic regions of the retinula or visual 

 cell show more or less marked changes in response to light and 

 darkness. The nucleus-bearing part of the visual cells some- 

 what widened in the dark. The refractive, middle segment 

 (analogous to an ellipsoid of a vertebrate retinula) disappears 

 in continuous illumination and is most conspicuous in eyes that 

 have been subjected to optimum illumination. The rhabdome 

 in light adapted eyes is a rounded cone-shaped body, while in 

 dark adapted material it is an elongated trough-shaped struc- 

 ture with its long dimension directed parallel to the axis of the 

 body of the animal. 



3. Despite the analogy that is apparent between the structure 

 of the retinula of a vertebrate and that of a Prorhynchus, there 

 is no analogy in functional changes shown. In the former it is 



